Personality genetics is a scientific field that examines the relationship between personality and genetics. Interest in the field is also driven by molecular genetic[1] and evolutionary ideas about personality.[2]

Most studies on personality genetics rely on twin studies, which compare identical and fraternal twins. Twin studies consistently indicate moderate genetic influence—heritability estimates for most traits are about 40% and are not influenced by the environmental factors shared by twins.[3] The estimated heritabilities for the personality measures were much lower than those obtained in studies of identical and fraternal twins, which suggests that twin studies have exaggerated the degree of genetic variation in personality.

Behavior genetics methods have similarities to natural experiments in which groups are found who have differences in genetic similarity or environmental similarity. The most popular method of determining human inheritance compares mono-zygotic and di-zygotic twins. Mono-zygotic twins develop from the same fertilized ovum and share the same set of inherited blueprints, often called identical twins. Another way to describe them is to say that their genotype is identical. Because mono zygotic twins are identical genetically, and observed differences between the twins can be noted to the environment.

Diygotic twins develop from two separate ova. They are no more alike genetically than other brothers and sisters; on average, they share 50% of their genes. Any observed differences between fraternal twins are attributable to a combination of genetic and environmental causes. The twin method assumes that environmental influences are essentially the same for the two types of twins. If the identical twins are observed to be more similar than they fraternal twins, this difference in their correlations is ascribed to the greater genetic similarity of the identical twins. This means that the trait is influenced by genetic factors.[4]

In order to examine the complications of developmental genetic influences on personality characteristics during the time of young adulthood, the study applied a longitudinal twin study. This classic twin design is a well-used method for determining the presence of genetic influences on individual differences. There were two different types of twins used in the study: monozygotic (identical) and dizygotic (fraternal) . Monozygotic twins share 100% of their genes, whereas dizygotic share only 50%. If there is a strong genetic influence, monozygotic co-twins have a tendency to be more similar than dizygotic twins.

A longitudinal design is a branch of this cross sectional twin design in that the twins are assessed twice in their life. A longitudinal study can find the presence of genetic influences at two different times and can determine the genetic influences that operate on a intra-individual change over a period of time. Longitudinal twin studies have been used repeatedly to investigate the behavioral and intellectual development in infants and young children. Most twin studies of adult personality characteristics have been cross-sectional. Cross-sectional studies do not show the full impact that developmental genetics has on adult personality. Even though cross-sectional methods have led to interesting findings,they are subject to associational effects and sample variation across different ages, and therefore cannot investigate intra-individual change.

This study assessed a large sample of mono-zygotic and dizygotic twins when they were 20 and 25 years of age, using a shorter version of the Minnesota Multiphasic Personality Inventory (MMPI). This inventory was used to measure self-reported personality characteristics of particular behavior of genetics interest. Three primary questions were developed during this assessment:

Does the importance of genetic influences on personality characteristics change across the 5 year period?

Are genetic influences important for the likeliness of co-twins to change in the same way over the period of time?

Are there genetic influences on the tendency of the co-twins to change, without keeping in mind of the direction of the change

Age differences create more variables even within a family, so the best comparisons are found using twins. Twins typically share a family environment called a shared environment because they may share other aspects like teachers, school, and friends. A nonshared environment means completely different environment for both subjects. "Biologically related children who are separated after birth and raised in different families live in nonshared environments." Identical twins separated at birth and raised in different families constitute the best cases for heredity and personality because similarities between the two are due only to genetic influences.[5]

Vulnerability was a factor in this study that was taken into consideration regarding the issue of genetic influences on vulnerability. The study concluded that the monozygotic co-twins would be more similar than dizygotic co-twins in change over time. To answer the questions as to whether change is genetically influenced through personality, the data concluded that there was no significant differences for either variances between the monozygotic and dizygotic co-twins.[6][7]

A link was found between the personality trait of neuroticism and a polymorphism called 5-HTTLPR in the serotonin transporter gene, but this association was not replicated in larger studies.[8] Other candidate gene studies have provided weak evidence that some personality traits are related to AVPR1A ("ruthlessness gene") and MAOA ("Warrior gene").[9]

Genotypes, or the genetic make up of an organism, influence, but don't fully decide the physical traits of a person. Those are also influenced by the environment and behaviors they are surrounded by. For example, a person's height is affected by genetics, but if they are malnourished growth will be stunted no matter what their genetic coding says. Environment is also not completely responsible for an outcome in personality. An example from Psychobiology of Personality by Marvin Zuckerman is alcoholism: studies suggest that alcoholism is an inherited disease, but if a subject with a strong biological background of alcoholism in their family tree is never exposed to alcohol, they will not get that way regardless of their genome.[5]

Another factor that can be addressed is biological versus adoptive relatives, a real-life experiment, adoption. This creates two groups: genetic relatives (biological parents and siblings) and environmental relatives (adoptive parents and siblings). So the question can be asked, are adopted children more like their biological parents, who share the same genes, or their adoptive parents, who share the same home environment? And consequently to sharing that home environment, do those adopted siblings come to share traits as well? After studying hundreds of adoptive families, the discovery was that people who grow up together, whether biologically related or not, do not much resemble one another in personality. In characteristics such as extroversion and agreeableness, adoptees are more like their biological parents than to their adoptive parents. However, the minute shared-environment effects does not mean that adoptive parenting is ineffective. Even though genetics may limit the family environment's influence on personality, parents do influence their children's attitudes, values, faith, manners and politics. In adoptive homes, child neglect and abuse and even divorce between the parents is uncommon. In accordance to that, it is not surprising, despite a somewhat greater risk of psychological disorder, most adopted children excel, especially when they're adopted as infants. In fact, seven out of eight have reported feeling a strong connection with one or even both of their adoptive parents.[10]